Method and apparatus for producing liquid products from air in various proportions

ABSTRACT

A cryogenic method and apparatus using a liquefier and a two stage distillation column capable of operating in two modes, namely a first mode of operation during which only liquid nitrogen is produced and a second mode of operation during which liquid nitrogen and liquid oxygen are produced. By adjusting the time of operation in each mode, any ratio of liquid nitrogen to liquid oxygen greater than the ratio achieved during the second mode of operation can be achieved. In the first mode of operation, a condenser is used to condense the lower pressure stage gaseous nitrogen into lower pressure stage nitrogen condensate. To condense the lower pressure stage gaseous nitrogen, either at least a portion of the crude oxygen liquid from the higher pressure stage, at least a portion of the oxygen-enriched liquid from the lower pressure stage, at least a portion of the liquefied air, or mixtures thereof, are introduced to the condenser. In the second mode of operation, the top condenser is not used; instead, all of the crude oxygen liquid is introduced into the lower pressure stage, which produces a bottom liquid oxygen stream and a low pressure overhead waste stream containing nitrogen. The system includes fluid flow lines and valves for directing the flow of certain fluids, particularly the crude oxygen liquid and the oxygen-enriched liquid, during the two modes of operation.

FIELD OF THE INVENTION

The present invention pertains to the production of liquid nitrogen as asingle product, or liquid nitrogen and liquid oxygen as two products, ina cryogenic air separation system.

BACKGROUND OF THE INVENTION

Liquefied atmospheric gases, e.g. oxygen, nitrogen, argon, etc., areincreasingly used in industry, providing cryogenic capabilities for avariety of industrial processes. As liquids, atmospheric gases are moreeconomical to transport and store in large quantities and provide readyand economical sources for gaseous products from liquid storagefacilities.

The production of liquefied atmospheric gases, particularly liquidnitrogen, requires more energy than the production of correspondinggaseous products because additional energy is required for liquefaction.Therefore, to meet the increasing needs for liquid atmospheric gases, itis desirable to develop a process which is energy efficient in operationand economical from a capital standpoint. Many various systems have beenused previously in an attempt to meet these needs.

For example, U.S. Pat. No. 3,605,422 discloses an air separation andliquefaction process, in which liquid nitrogen and liquid oxygen areproduced directly from a two stage distillation column. A nitrogenrecycle refrigeration system is used to provide sufficient refrigerationto produce liquids. Nonetheless, this process is capital intensive.

British Patent No. 1,472,402 discloses a cryogenic air separation cyclein which gaseous nitrogen is withdrawn from a distillation column, isliquefied in a separate system, and is subsequently partially recoveredas a product and partially recycled to the distillation column asreflux.

U.S. Pat. No. 4,152,130 discloses a process for producing liquidnitrogen and liquid oxygen by the cryogenic separation of air using atwo stage distillation column and an air recycle liquefaction system.Gaseous and liquid air are delivered to the high pressure stage of thedistillation column as feeds. Liquid nitrogen is withdrawn from thereboiler/condenser of the high pressure stage of the distillationcolumn, and liquid oxygen is derived from the sump of the low pressurestage of the column. A liquid fraction is also withdrawn from the highpressure stage of the column and is ultimately used as reflux for thelow pressure stage of the column. The removal of liquid nitrogen as aproduct directly from the high pressure stage of the distillation columnreduces the amount of available reflux in the low pressure stage of thecolumn, which limits liquid product recoveries. U.S. Pat. No. 4,375,367discloses a process derived from the '130 patent which requires lesscapital expenditure due to the elimination of a tandem companderapparatus.

U.S. Pat. No. 4,715,873 discloses a cycle wherein at least a portion ofthe liquid feed air bypasses the distillation column and is used toliquefy the gaseous products of the column. The resulting vapor airstream is retained at elevated pressure.

U.S. Pat. No. 5,355,681 discloses a process for the separation of airinto its components using a distillation column system having at leasttwo distillation columns. A portion of the feed air is condensed and atleast a portion of this liquefied air is used as impure reflux in one ofthe distillation columns. A waste stream is removed from a locationsituated no more than four theoretical stages above the location wherethe liquefied air is fed to one of the columns.

In these and other known prior art processes, liquid nitrogen and liquidoxygen with high recovery can typically provide only certain relativeamounts of the two products. These relative amounts are not alwaysconsistent with current demand. Therefore, there is a need for greaterflexibility in the relative amounts of liquid nitrogen and liquid oxygenproduced, without sacrificing any power.

More specifically, demand for liquid oxygen and liquid nitrogen changes(sometimes unpredictably) over time. A liquefier with a full recovery ofnitrogen and oxygen from air cannot usually satisfy market needs overthe life of a given plant, because total plant production is limited bythe size of the plant and because the ratio of liquid nitrogen producedto liquid oxygen produced is, in part, determined by air composition.Therefore, an existing full recovery liquefier is only able to match ademand for one of its products (either nitrogen or oxygen) producing atthe same time too little or too much of the other product. Moreover, aplant cannot continue to produce too much of one of its cryogenicliquids without being able to sell it, because of the high power costand limited storage capacity. This leads to the need to reduce the totalproduction of the plant (i.e., "turn down"), which is highlyuneconomical and undesirable.

SUMMARY OF THE INVENTION

The present invention is directed to a method for operating a cryogenicdistillation column having a higher pressure stage and a lower pressurestage to produce liquid nitrogen alone or liquid nitrogen and liquidoxygen. The present invention is also directed to a system capable ofoperating in two modes, namely a first mode of operation during whichonly liquid nitrogen is produced and a second mode of operation duringwhich liquid nitrogen and liquid oxygen are produced.

According to a first embodiment of the present invention, a cryogenicdistillation column having a higher pressure stage and a lower pressurestage is operated to produce only liquid nitrogen. A liquefier providesa stream of cooled gaseous feed air and a stream of liquefied air. Thecooled gaseous feed air is introduced into the higher pressure stage forrectification into a high pressure nitrogen overhead at the top of thehigher pressure stage and a crude oxygen liquid at the bottom of thehigher pressure stage. The high pressure nitrogen is condensed by heatexchange with an oxygen-enriched liquid from the bottom of the lowerpressure stage. A portion of the condensed nitrogen is used as reflux tothe higher pressure stage and the remaining portion of the condensednitrogen is withdrawn as liquid nitrogen product. The liquefied air maybe cooled, and at least a portion of the liquefied air is introduced tothe lower pressure stage to be separated into lower pressure stagegaseous nitrogen at the top of the lower pressure stage and anoxygen-enriched liquid at the bottom of the lower pressure stage. Atleast a portion of the crude oxygen liquid, at least a portion of theoxygen-enriched liquid, at least a portion of the cooled liquefied air,or mixtures of any of these three liquids may be introduced into acondenser of the lower pressure stage to condense the lower pressurestage gaseous nitrogen to form lower pressure stage nitrogen condensate.In a preferred embodiment, a stream including: (i) at least a portion ofthe crude oxygen liquid and (ii) at least a portion of at least one ofthe oxygen-enriched liquid and the liquefied air, is introduced to thecondenser of the lower pressure stage, as opposed to any of these threestreams or mixtures thereof. A portion of the lower pressure stagenitrogen condensate is utilized as reflux for the lower pressure stage,while the remaining portion of the lower pressure stage nitrogencondensate is withdrawn as liquid nitrogen product.

According to another embodiment of the present invention, the cryogenicdistillation column is used to produce liquid nitrogen and liquidoxygen. Optionally, argon can also be produced in this embodiment. Bothproducts are produced by varying the mode of production of the cryogenicprocess between a first mode of production during which only liquidnitrogen is produced and a second mode of operation during which liquidnitrogen and liquid oxygen are produced. The process during the firstmode of operation is identical to the method described above. The secondmode of operation is similar to the first mode of operation in that theliquefier is used to produce a stream of cooled gaseous feed air and astream of liquefied air. Also similar to the first mode of operation,the cooled gaseous feed air is fed into the higher pressure stage forrectification into a high pressure nitrogen overhead and a crude oxygenliquid, and the high pressure nitrogen is condensed with some of it usedas reflux to the higher pressure stage. In the second mode of operation,however, the condenser of the lower pressure stage is not used; instead,the crude oxygen liquid is cooled and introduced into the lower pressurestage. The liquefied air is also cooled and introduced to the lowerpressure stage at a location different from where the crude oxygenliquid is introduced. The lower pressure stage produces a lower pressureoverhead waste stream containing nitrogen (as well as oxygen and argon)and the oxygen-enriched liquid, which is a product liquid oxygen streamin this mode of operation. The product oxygen liquid is cooled againstthe crude liquid oxygen stream before the crude oxygen liquid isintroduced to the lower pressure stage.

The present invention also includes a system, capable of operating inthe two modes of operation, for producing liquid nitrogen and liquidoxygen, and optionally argon. The system includes the liquefier and thetwo stage distillation column having a reboiler/condenser for condensingthe high pressure nitrogen from the higher pressure stage by heatexchange with the oxygen-enriched liquid from the bottom of the lowerpressure stage. As in the processes described above, a lower pressurestage separates at least a portion of the cooled liquefied air intolower pressure stage gaseous nitrogen and an oxygen-enriched liquid. Atop condenser condenses the lower pressure stage gaseous nitrogenselectively, namely only during the first mode of operation. In oneembodiment, the system includes a first set of fluid flow lines andvalves extending between the bottom of the higher pressure stage, thecondenser, and the lower pressure stage, for permitting crude oxygenliquid to flow from the bottom of the higher pressure stage to: (i) thecondenser during the first mode of operation, and (ii) the lowerpressure stage during the second mode of operation. The system alsoincludes a second set of fluid flow lines and valves extending betweenthe bottom of the lower pressure stage, a liquid oxygen product storage,and the condenser, for permitting the oxygen-enriched liquid to flowfrom the bottom of the lower pressure stage to: (i) the condenser duringthe first mode of operation, and (ii) the liquid oxygen product storageduring the second mode of operation. A third set of fluid flow lines andvalves may be employed as an alternative to the second set of fluid flowlines and valves. The third set of fluid flow lines and valves extendsbetween two positions near the bottom of the lower pressure stage, theliquid oxygen product storage, and a waste stream, for permitting: (i) abottom vapor waste stream to flow from a first position near the bottomof the lower pressure stage to the vapor waste stream during the firstmode of operation and (ii) the oxygen-enriched liquid to flow from asecond position, below said first position, near the bottom of saidlower pressure stage to the liquid oxygen product storage during thesecond mode of operation.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an embodiment of the present invention;

FIG. 1A is fragmentary view of the embodiment shown in FIG. 1 showing,as solid lines, the fluid flow lines in operation during the first modeof operation in which liquid nitrogen is produced and showing theremaining fluid flow lines as dashed lines;

FIG. 1B is a fragmentary view of the embodiment shown in FIG. 1 showing,as solid lines, the fluid flow lines in operation during the second modeof operation in which both liquid nitrogen and liquid oxygen areproduced and showing the remaining lines as dashed lines;

FIG. 2 is a schematic diagram of a second embodiment of the presentinvention;

FIG. 3 is a schematic diagram of a third embodiment of the presentinvention;

FIG. 4 is a schematic diagram of a fourth embodiment of the presentinvention; and

FIG. 5 is a schematic diagram of a fifth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to an air liquefaction and air separationcycle capable of operation in at least two modes:

1) a first mode of operation, during which only liquid nitrogen isproduced; and

2) a second mode of operation, during which liquid nitrogen and liquidoxygen are produced simultaneously.

The second mode of operation can be designed at any ratio of liquidnitrogen produced to liquid oxygen produced (hereinafter referred to a"LIN/LOX ratio"). A smaller LIN/LOX ratio in the second mode ofoperation provides for a wider range of overall production ratios. (Anoverall production ratio is defined as the time-averaged LIN/LOX ratioproduced over a designated period of time.) Therefore, liquid oxygenproduction should be maximized in the second mode of operation. Thecycle proposed in the present invention can efficiently produce liquidnitrogen and liquid oxygen at LIN/LOX ratio of 1:1 in the second mode ofoperation. Accordingly, in such a system, an overall production ratiocan be anything greater than or equal to 1:1.

A desired overall production LIN/LOX ratio is achieved by running theplant in the two operating modes for different time intervals. If t₁ isthe number of days of operation in the first mode of operation and t₂ isthe number of days in the second mode of operation, then these timeintervals should obey the following relation: ##EQU1## While therelative values of t₁ and t₂ are given in the above equation, theabsolute values will be dictated by the size of the liquid nitrogen andliquid oxygen storage tanks. The switch from one mode to the othershould be performed such that the liquid levels in either of the tanksnever exceed the acceptable limits.

Referring now to the drawing, wherein like reference numerals refer tolike elements throughout, FIG. 1 shows a preferred embodiment of thepresent invention using an air liquefier 11 and a two-stage cryogenicdistillation column. Any type of known liquefier can be used, such as anair liquefier, a nitrogen liquefier, or a hybrid thereof (i.e., acombination of an air liquefier and a nitrogen liquefier). In addition,any known air liquefier can be used with various combinations of two orthree expanders at high or low pressure, for example a three expander,high pressure liquefier as disclosed in U.S. Pat. No. 4,894,076.

For purposes of simplicity in discussing the present invention, astandard two-compander air liquefier 11 is shown. Feed air is introducedin feed air line 10, compressed in main air compressor 12, after cooledin heat exchanger 14, cleaned of water and carbon dioxide in anadsorption unit 16 (preferably a molecular sieve adsorption unit), andcombined with a recycle air stream in line 74 to form a combined airstream in line 18. The combined air stream in line 18 is furthercompressed in recycle compressor 20, after cooled in heat exchanger 22,and split into two streams in lines 26 and 28 which are respectivelycompressed again in companders 30 and 32. The streams from lines 34 and36, which are respectively associated with companders 30 and 32, arecombined to form a combined stream in line 38, which is subsequentlyafter cooled in heat exchanger 40 against an external cooling fluid. Theresulting stream in line 42 is split into two streams in lines 44 and46.

Stream in line 46 is expanded in an expansion turbine 48 to a lowerpressure and temperature in line 50, which is then combined with thereturning recycle air stream in line 70 to form a combined stream inline 72. Stream in line 72 is passed through a warm stage 52 of a mainheat exchanger 51 to result in recycle air stream in line 74. Stream inline 44 is cooled in the warm stage 52 of the main heat exchanger 51before being split into a first stream in line 58 and a second stream inline 60.

First stream in line 58 is cooled in a cold stage 68 of the main heatexchanger 51 leading to a cooled stream in line 76, reduced in pressureacross an isenthalpic Joule-Thompson (JT) valve 77, and then flashed ina separator 90 providing feed liquefied air in line 134 for thedistillation system and a vapor flash stream in line 132. Second streamin line 60 is expanded in an expansion turbine 62 to a lower temperatureand pressure resulting in stream line 64 and then split into two streamsin lines 66 and 78.

Stream in line 66 is returned through the cold stage 68 of the main heatexchanger 51 leading to cooled stream in line 70 which is combined withstream in line 50 to form combined stream in line 72. The combinedstream in line 72 is then led through the warm stage 52 of the main heatexchanger 51 to form the recycle stream in line 74, as discussed above.Stream in line 78 is combined with vapor flash stream 132 and theresulting steam in line 80 is introduced as a cooled gaseous feed air tothe higher pressure stage 82 of the distillation column 81.

Higher pressure stage 82 of the distillation column 81 rectifies thecooled gaseous feed air into a high pressure nitrogen overhead vapor atthe top of the higher pressure stage 82 and a crude oxygen liquid at thebottom of the higher pressure stage 82. The high pressure nitrogenoverhead vapor is condensed in a reboiler/condenser 84 by heat exchangewith an oxygen-enriched liquid from the bottom of a lower pressure stage86 of distillation column 81. Reboiler/condenser 84 may be containedwithin and located at the bottom of lower pressure stage 86 as shown ormay be located outside of lower pressure stage 86 or elsewhere. Aportion of the condensed nitrogen provides reflux to higher pressurestage 82. The remaining portion of the condensed nitrogen is withdrawnvia line 110. Although the stream in line 110 may be withdrawn asnitrogen product directly, FIG. 1 shows an embodiment in which stream inline 110 is further processed prior to removal as product as discussedin detail below.

In the first mode of operation during which only liquid nitrogen isproduced as a product (as best shown in FIG. 1A), the operating pressurein lower pressure stage 86 is about 0.32 MPa. Liquefied feed air in line134 is cooled, for example in a sub-cooler 94, against a combined vaporwaste stream in line 158. All of the liquefied feed air may then beintroduced to the lower pressure stage 86 or, as shown, stream in line136 may be split into two portions, stream in line 140 and stream inline 138. Stream in line 140 is expanded across a JT valve andintroduced into lower pressure stage 86, where the liquefied air isseparated into the lower pressure stage gaseous nitrogen at the top oflower pressure stage 86 and the oxygen-enriched liquid at the bottom oflower pressure stage 86 leading to stream in line 104. A portion ofliquefied air in line 134 can also be introduced to the higher pressurestage 82 (not shown).

Crude oxygen liquid from higher pressure stage 82 is fed to line 92,sub-cooled in heat exchanger 94 resulting in stream in line 96,sub-cooled further in heat exchanger 112 (again preferably against acombined vapor waste stream in line 156), reduced in pressure across aJT valve, combined with the portion of liquefied air stream in line 138resulting in stream in line 146, and combined with the oxygen-enrichedbottom product from the lower pressure stage 86 in line 108. Theresulting stream in line 148 is introduced to a condenser 88 of lowerpressure stage 86, where it is vaporized and used to condense the lowerpressure stage gaseous nitrogen to form a lower pressure stage nitrogencondensate. Alternatively, either a portion or all of sub-cooled crudeoxygen liquid in line 96 could be fed to lower pressure stage 86 vialine 102 and later withdrawn as oxygen-enriched liquid in line 104 anddirected to condenser 88.

In the first mode of operation, liquid nitrogen product may be withdrawndirectly as shown from streams in lines 122 and 110. The process shownin FIG. 1A is an alternative method to direct withdrawal. As shown inFIG. 1A, the remaining portion of the condensed nitrogen (which is notused as reflux) in line 110 is sub-cooled in heat exchanger 112 toresult in stream in line 114 and reduced in pressure across a JT valvethen flashed in a phase separator 116 to form first low pressure vapornitrogen in line 120 and low pressure liquid nitrogen in line 118. Lowpressure vapor nitrogen is introduced via line 120 to the lower pressurestage 86 near the top of lower pressure stage 86. Low pressure liquidnitrogen stream in line 118 is reduced in pressure then further reducedin pressure across a JT valve and separated in phase separator 126 toform second low pressure vapor nitrogen in line 128 and the liquidnitrogen product in line 130, which may be directed to a liquid nitrogenstorage tank (not shown).

As shown in FIG. 1A, the remaining portion of the lower pressure stagenitrogen condensate (which is not used as reflux) in line 122 iscombined with low pressure liquid nitrogen after it is initiallypressure reduced. Also, the second low pressure vapor nitrogen in line128 is combined with the oxygen-enriched vapor waste stream in line 154from condenser 88 to form combined vapor waste stream 156 which is usedas a refrigerant to cool the crude oxygen liquid, the liquefied air, andthe remaining portion of the condensed nitrogen (which is not used asreflux) from higher pressure stage 82. More specifically, stream in line156 is first introduced to heat exchanger 112 to sub-cool the remainingportion of the condensed nitrogen in line 110 and crude oxygen liquid inline 96 resulting in stream in line 158. Stream in line 158 is then usedto cool crude oxygen liquid in line 92 and liquefied air in line 134resulting in stream in line 160. Stream in line 160 is used as arefrigerant for the main heat exchanger 51. Specifically, stream in line160 is fed to the cold stage 68 of the main heat exchanger 51 resultingin stream in line 162, which is fed to the warm stage 52 of the mainheat exchanger 51 resulting in waste stream in line 164, which is ventedto atmosphere.

In the second mode of operation during which liquid nitrogen and liquidoxygen are produced as products (as best shown in FIG. 1B), theoperating pressure in lower pressure stage 86 is about 0.13 MPa. Crudeoxygen bottom liquid in line 92 is sub-cooled in heat exchanger 94 andreduced in pressure across a JT valve. The resulting stream in line 98is passed through liquid oxygen sub-cooler 100 providing necessaryrefrigeration for liquid oxygen product 106 and introduced in theappropriate location as a feed in line 102 to the lower pressure stage86 of the distillation column 81. The liquefied feed air in line 134 issub-cooled, for example in a heat exchanger 94, against a combined vaporwaste stream in line 158. The resulting stream in line 136 is thenreduced in pressure across a JT valve and fed to the lower pressurestage 86 at a location that is different from the crude oxygen liquidfeed location.

In the second mode of operation, all of the liquefied air is directed tostream in line 142 and introduced into lower pressure stage 86. Aportion of the liquefied air may be introduced into higher pressurestage 82 (not shown). The various feeds to lower pressure stage 86 aredistilled to produce a low pressure vapor overhead waste stream in line152, which is warmed up in heat exchangers 112, 94, 68, 52 and vented,and oxygen-enriched liquid in line 104, which is sub-cooled in heatexchanger 100 against the crude oxygen liquid in line 98 and withdrawnas product in line 106. Thus, as shown in FIG. 1B, the low pressureoverhead waste stream in line 152 is used to cool the remaining portionof the condensed nitrogen from higher pressure stage 82 in line 110, theliquefied air in line 134, and the crude oxygen liquid in line 92. Inthe second mode of operation, the top condenser 88 is not used.

If necessary, argon can also be produced in the second mode ofoperation. This would involve an additional side-rectifier connected byliquid and vapor streams to the lower pressure stage 86. This option isnot shown in the figures, but it is well-known in the art.

As discussed above, when an overall production LIN/LOX ratio is desired,the times of operation in the first mode and the second mode areselected so that the time-averaged, desired overall production LIN/LOXratio is achieved. The weight ratio achieved during the second mode ofoperation is also a factor in determining the relative times ofoperation in the two modes. In one embodiment, the LIN/LOX ratio in thesecond mode of operation is 1:1, although this ratio will depend on theliquid/vapor flow rates in each stage, the numbers of theoretical traysin each stage, and the feed composition. In this embodiment, any overallproduction LIN/LOX ratio greater than or equal to 1:1 can be achieved;for example the overall production LIN/LOX ratio can be infinity byoperating exclusively in the first mode of operation or can be 1:1 byoperating exclusively in the second mode of operation.

The system of the present invention for producing liquid nitrogen andliquid oxygen includes liquefier 11, which provides a stream of cooledgaseous feed air in line 80 and a stream of liquefied air in line 134,and distillation column 81 which has higher pressure stage 82 and lowerpressure stage 86. The system also includes a first set of fluid flowlines 92, 98, 102, 146, 148 and valves, disposed in these lines,extending between the bottom of higher pressure stage 82, condenser 88,and lower pressure stage 86, for permitting crude oxygen liquid to flowfrom the bottom of higher pressure stage 82 to: (i) condenser 88 duringthe first mode of operation; and (ii) lower pressure stage 86 during thesecond mode of operation. For example, during the first mode ofoperation, the valve disposed between lines 96 and 98 is closed and thevalve disposed between lines 96 and 146 is open. In the second mode ofoperation, the positions of these two valves are reversed.Alternatively, crude oxygen liquid, or a portion thereof, can bedirected to lower pressure stage 86 also during the first mode ofoperation. It is later withdrawn as oxygen-enriched liquid in line 104and directed to condenser 88 via line 108.

The system also includes a second set of fluid flow lines 104, 106, 108,148 and valves, disposed in these lines, extending between the bottom oflower pressure stage 86, a liquid oxygen product storage 106 (such as atank), and condenser 88, for permitting oxygen-enriched liquid to flowfrom the bottom of lower pressure stage 86 to: (i) condenser 88 duringthe first mode of operation; and (ii) liquid oxygen product storage vialine 106 during the second mode of operation. For example, during thefirst mode of operation, the valve disposed between lines 104 and 106 isclosed and the valve disposed between lines 104 and 108 is open. In thesecond mode of operation, the positions of these two valves arereversed. It should be noted that some of these lines may overlap oneanother; for example line 148 can be used as part of both the first andsecond sets of fluid flow lines and valves.

As an alternative to the second set of fluid flow lines and valves, thesystem may include a third set of fluid flow lines 200, 104, 106 andvalves (as shown in FIGS. 2-4). This third set extends between thebottom of lower pressure stage 86, a liquid oxygen product storage, anda vapor waste stream in line 158 (as in FIG. 2) or 156 (as in FIGS. 3and 4), for permitting: (i) a bottom vapor waste stream to flow from afirst position near the bottom of lower pressure stage 86 to theappropriate vapor waste stream during the first mode of operation; and(ii) the oxygen-enriched liquid to flow from a second position, belowsaid first position, near the bottom of said lower pressure stage toliquid oxygen product storage during the second mode of operation. Thefirst and second positions are selected such that primarily vapor iswithdrawn at the first position and primarily liquid is withdrawn at thesecond position. During the first mode of operation, the valve disposedbetween lines 104 and 106 is closed and the valve disposed between lines200 and 158 (as in FIG. 2) or 156 (as in FIGS. 3 and 4) is open. In thesecond mode of operation, the positions of these two valves arereversed.

The processes using the systems depicted in FIGS. 2-4 are directed tovariations in the first mode of operation. As shown in FIG. 2, a bottomvapor waste stream is withdrawn in line 200 instead of removing theliquid waste stream in line 104 from the lower pressure stage 86 anddelivering it to condenser 88, as is done in the embodiment shown inFIGS. 1 and 1A. During the first mode of operation in the embodimentsshown in FIGS. 2-4, the step of introducing a mixture to condenser 88includes introducing a portion of the crude oxygen liquid and a portionof the liquefied air to condenser 88. In these embodiments, theremaining portions of the crude oxygen liquid and the liquefied air areintroduced to lower pressure stage 88, and a vapor waste stream iswithdrawn in line 200 from the bottom of lower pressure stage 86.

In the embodiment shown in FIG. 2, vapor waste stream in line 200 isreduced in pressure across a JT valve and combined with theoxygen-enriched vapor waste stream in line 158 from condenser 88. Theresulting stream forms a combined vapor waste stream which is used as arefrigerant to cool the crude oxygen liquid and the liquefied air inheat exchanger 94. This embodiment permits the pressure of lowerpressure stage 86 to be reduced from about 0.32 MPa to about 0.24 MPa,although the recovery of liquid nitrogen from the lower pressure stageslightly decreases.

FIG. 3 shows another embodiment of the present invention directedprimarily to the first mode of operation. As in the embodiment shown inFIG. 2, vapor waste stream in line 200 is withdrawn from the lowerpressure stage 86. Vapor waste stream in line 200 is then expanded in anexpander 202 to a lower pressure and combined with the oxygen-enrichedvapor waste stream in line 154 from condenser 88. The resulting streamin line 156 forms a combined vapor waste stream which is used as arefrigerant to cool the crude oxygen liquid, the liquefied air, and theremaining portion of the condensed nitrogen from higher pressure stage82, in heat exchangers 112 and 94. In this embodiment, the pressure inlower pressure stage 86 remains at about 0.24 MPa, but recovery ofnitrogen increases compared to the embodiment shown in FIG. 2.

FIG. 4 shows yet another embodiment of the present invention directedprimarily to the first mode of operation. As in the embodiments shown inFIGS. 2 and 3, vapor waste stream in line 200 is withdrawn from thelower pressure stage 86. Vapor waste stream in line 200 is then directedto an eductor 204, where it is reduced in pressure and combined with theoxygen-enriched vapor waste stream from condenser 88. Eductor 204 alsoserves to reduce the pressure of the oxygen-enriched vapor waste streamin line 154 and, consequently, of condenser 88 via line 150. Theresulting stream in line 156 forms a combined vapor waste stream whichis used as a refrigerant to cool the crude oxygen liquid, the liquefiedair, and the remaining portion of the condensed nitrogen from higherpressure stage 82, in heat exchangers 112 and 94.

FIG. 5 shows another alternative embodiment of the present invention foruse when power cost varies depending on the time of the day. In thiscase, the liquefaction system has been intentionally oversized toproduce an excess mount of liquefied air during hours when the cost ofpower is relatively low. Excess liquefied air is stored in storage tank300, which is disposed between liquefier 11 and distillation column 81.Excess liquefied air is stored during a first time period when the costof power is relatively lower. At least a portion of the excess air isused during a second period of time when the cost of power is relativelyhigher, at which time liquefaction system may be turned off; during thetime when the liquefaction system is off, the required gaseous air issupplied from the main air compressor.

EXAMPLES

In order to demonstrate the efficacy of the present invention and toprovide a comparison to a conventional process, the following exampleswere developed. In Table 1 below, the power required for the proposedcycle has been calculated for a 600 ton/day liquefier, assumingisothermal efficiency for main compressor 12 and recycle compressor 20of 70%, isentropic efficiency for compander compressor 30, 32 of 83%,and isentropic efficiency for expanders 48, 62 of 89%. For comparison,the power required by a conventional full recovery nitrogen recycle,producing 600 tons/day of liquids at a fixed LIN/LOX ratio of 2.5, hasalso been determined. The power required by the conventional fullrecovery nitrogen recycle was about 2% higher than the power required bythe present invention at the same LIN/LOX ratio, namely 11,818 kW versus11,572 kW.

                  TABLE 1                                                         ______________________________________                                        Power of Compared Liquefiers at a Production Rate 600 t/day                   CYCLE            LIN/LOX weight ratio                                                                        Power  kW!                                     ______________________________________                                        Present Invention, second mode                                                                 1.2           11,643                                         Present Invention, first mode                                                                  ∞       11,454                                         Full recovery, nitrogen recycle                                                                2.5           11,818                                         ______________________________________                                    

Some of the stream parameters of simulations are shown in Tables 2 and3. The basis of the simulations is the production of 600 ton/day ofliquid product, namely 600 ton/day of liquid nitrogen in the case ofTable 2 and 600 ton/day of total liquid including liquid nitrogen andliquid oxygen in the case of Table 3. The feed used in the simulationswas atmospheric air at the pressure and temperature shown in Tables 2and 3 for stream in line 10. In the simulations, the number oftheoretical trays in the higher pressure stage was 40 and the number oftheoretical trays in the lower pressure stage was 73.

In the simulation reported in Table 2, the product liquid nitrogencontained 2 ppm of oxygen, and the waste stream in line 164 had acomposition of 61.64% nitrogen and 36.73% oxygen, along with some argon.

In the simulation reported in Table 3, the product liquid nitrogencontained 2 ppm of oxygen, and the purity of liquid oxygen produced was99.50%. The waste stream in line 164 had a composition of 89.82%nitrogen and 8.85% oxygen, along with some argon.

                  TABLE 2                                                         ______________________________________                                        Stream Parameters for the Embodiment shown in FIG. 1                          during the First Mode of Operation (also shown in FIG. 1A)                                          Flow Rate                                               Stream     Temperature                                                                              Pressure  (lbmol/                                       in Line Number                                                                           (°F.)                                                                          (K.)   (psi)                                                                              (kPa)                                                                              hour) gmole/s                             ______________________________________                                        10         80.0    299.8  14.7 101.4                                                                              4188.8                                                                              527.8                               132        -280.2  99.7   94.0 648.1                                                                              38.8  4.9                                 78         -276.7  101.7  93.1 641.9                                                                              2139.7                                                                              269.6                               134        -280.1  99.8   94.0 648.1                                                                              1977.6                                                                              249.2                               140        -283.0  98.2   93.0 641.2                                                                              1684.6                                                                              212.3                               142        -290.9  93.8   60.0 413.7                                                                              1684.6                                                                              212.3                               138        -283.0  98.2   93.0 641.2                                                                              293.0 36.9                                92         -277.1  101.4  93.0 641.2                                                                              1235.1                                                                              155.6                               96         -283.0  98.2   92.0 634.3                                                                              1235.1                                                                              155.6                               146        -290.0  94.3   91.0 627.4                                                                              1528.1                                                                              192.5                               104        -287.6  95.6   50.9 350.9                                                                              657.1 82.8                                110        -285.6  96.7   89.1 614.3                                                                              943.4 118.9                               114        -290.0  94.3   88.1 607.4                                                                              943.4 118.9                               118        -299.4  89.0   48.0 330.9                                                                              879.9 110.9                               120        -299.4  89.0   48.0 330.9                                                                              63.5  8.0                                 122        -299.8  88.8   47.0 324.1                                                                              1091.1                                                                              137.5                               128        -315.5  80.1   20.0 137.9                                                                              185.2 23.3                                130        -315.5  80.1   20.0 137.9 1785.7                                                                       225.0                                     150        -302.5  87.3   20.0 137.9                                                                              2185.2                                                                              275.3                               156        -303.8  86.6   19.0 131.0                                                                              2370.5                                                                              298.7                               158        -293.9  92.1   18.0 124.1                                                                              2370.5                                                                              298.7                               160        -283.7  97.8   17.0 117.2                                                                              2370.5                                                                              298.7                               164        82.9    301.4  15.0 103.4                                                                              2370.5                                                                              298.7                               ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Stream Parameters for the Embodiment shown in FIG. 1                          during the Second Mode of Operation (also shown in FIG. 1B)                                         Flow Rate                                                        Temperature                                                                            Pressure  (lbmol/                                           Stream Number                                                                            (°F.)                                                                          (K.)   (psi)                                                                              (kPa)                                                                              hour) gmole/s                             ______________________________________                                        10         80.0    299.8  14.7 101.4                                                                              4619.0                                                                              581.98                              132        -280.2  99.7   95.0 655.0                                                                              20.4  2.57                                78         -276.7  101.7  90.2 621.9                                                                              2637.1                                                                              332.27                              134        -280.1  99.8   95.0 655.0                                                                              1929.4                                                                              243.10                              136        -290.0  94.3   94.0 648.1                                                                              1929.4                                                                              243.10                              142        -308.4  84.0   25.0 172.4                                                                              1929.4                                                                              243.10                              92         -277.1  101.4  93.1 641.9                                                                              1513.0                                                                              190.63                              96         -290.0  94.3   92.1 635.0                                                                              1513.0                                                                              190.63                              98         -305.6  85.6   25.0 172.4                                                                              1513.0                                                                              190.63                              102        -306.3  85.2   24.0 165.5                                                                              1513.0                                                                              190.63                              104        -287.6  95.6   25.1 173.1                                                                              714.0 89.96                               106        -292.6  92.8   24.1 166.2                                                                              714.0 89.96                               110        -285.6  96.7   89.2 615.0                                                                              1144.5                                                                              144.20                              114        -289.7  94.4   88.2 608.1                                                                              1144.5                                                                              144.20                              128        -314.6  80.6   21.2 146.2                                                                              176.3 22.21                               130        -314.6  80.6   21.2 146.2                                                                              968.2 121.99                              152        -310.5  82.9   21.2 146.2                                                                              2728.4                                                                              343.77                              156        -310.9  82.7   20.7 142.7                                                                              2904.7                                                                              365.98                              158        -308.1  84.2   19.7 135.8                                                                              2904.7                                                                              365.98                              160        -283.0  98.2   18.7 128.9                                                                              2904.7                                                                              365.98                              164        80.1    299.8  15.7 108.2                                                                              2904.7                                                                              365.98                              ______________________________________                                    

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A method of operating a cryogenic distillationcolumn having a higher pressure stage and a lower pressure stage toproduce liquid nitrogen comprising the steps of: (a) using a liquefierto provide a stream of cooled gaseous feed air and a stream of liquefiedair; (b) introducing said cooled gaseous feed air into said higherpressure stage of said distillation column for rectification into a highpressure nitrogen overhead at the top of said higher pressure stage anda crude oxygen liquid at the bottom of said higher pressure stage; (c)condensing said high pressure nitrogen from said higher pressure stageby heat exchange with an lo oxygen-enriched liquid from the bottom ofsaid lower pressure stage of said distillation column; (d) utilizing aportion of said condensed nitrogen as reflux to said higher pressurestage of said distillation column; (e) introducing at least a portion ofsaid liquefied air to said lower pressure stage to separate saidliquefied air in said lower pressure stage into lower pressure stagegaseous nitrogen at the top of said lower pressure stage and saidoxygen-enriched liquid at the bottom of said lower pressure stage;characterized in that said method further comprises the steps of: (f)introducing a stream including: (i) at least a portion of said crudeoxygen liquid and (ii) at least a portion of at least one of saidoxygen-enriched liquid and said liquefied air, to a condenser of saidlower pressure stage to condense said lower pressure stage gaseousnitrogen to form a lower pressure stage nitrogen condensate; andutilizing a portion of said lower pressure stage nitrogen condensate asreflux to said lower pressure stage and withdrawing the remainingportion of said lower pressure stage nitrogen condensate and theremaining portion of said condensed nitrogen as liquid nitrogen product.2. The method of claim 1, wherein the step of introducing a stream tothe condenser of said lower pressure stage includes introducing all ofsaid crude oxygen liquid, all of said oxygen-enriched liquid, and aportion of said liquefied air to the condenser of said lower pressurestage.
 3. The method of claim 1, wherein:the step of introducing astream to the condenser of said lower pressure stage comprisesintroducing a portion of said crude oxygen liquid and a portion of saidliquefied air to said condenser of said lower pressure stage; and saidmethod further comprises the step of introducing the remaining portionof said crude oxygen liquid to said lower pressure stage.
 4. The methodof claim 3 further comprising the step of withdrawing a vapor wastestream from the bottom of said lower pressure stage.
 5. The method ofclaim 4 further comprising the step of isenthalpically reducing thepressure of the vapor waste stream from the bottom of said lowerpressure stage and combining said vapor waste stream with anoxygen-enriched vapor waste stream from said condenser of said lowerpressure stage to form a combined vapor waste stream which is used as arefrigerant to sub-cool said crude oxygen liquid and said liquefied air.6. The method of claim 4 further comprising step of isentropicallyreducing the pressure of the vapor waste stream from the bottom of saidlower pressure stage by using an expander and combining said vapor wastestream with an oxygen-enriched vapor waste stream from said condenser ofsaid lower pressure stage to form a combined vapor waste stream which isused as a refrigerant to cool said crude oxygen liquid, said liquefiedair, and the remaining portion of said condensed nitrogen from saidhigher pressure stage.
 7. The method of claim 4 further comprising thestep of pressure reducing said vapor waste stream and combining saidvapor waste stream with an oxygen-enriched vapor waste stream from saidcondenser of said lower pressure stage in an eductor to form a combinedvapor waste stream which is used as a refrigerant to cool said crudeoxygen liquid, said liquefied air, and the remaining portion of saidcondensed nitrogen from said higher pressure stage.
 8. The method ofclaim 1, wherein the step of withdrawing the remaining portion of saidcondensed nitrogen from said higher pressure stage as liquid nitrogenproduct includes the steps of:cooling said remaining portion of saidcondensed nitrogen against an oxygen-enriched vapor waste stream fromsaid condenser of said lower pressure stage; phase separating saidcooled condensed nitrogen in a first separator to form first lowpressure vapor nitrogen and low pressure liquid nitrogen; and phaseseparating said low pressure liquid nitrogen in a second separator toform second low pressure vapor nitrogen and said liquid nitrogenproduct.
 9. The method of claim 8 further comprising the stepsof:introducing said first low pressure vapor nitrogen to the top of saidlower pressure stage; combining the remaining portion of said lowerpressure stage nitrogen condensate with said low pressure liquidnitrogen; and combining said second low pressure vapor nitrogen with anoxygen-enriched vapor waste stream from said condenser of said lowerpressure stage to form a combined vapor waste stream which is used as arefrigerant to cool said crude oxygen liquid, said liquefied air, andthe remaining portion of said condensed nitrogen from said higherpressure stage.
 10. A method of operating a cryogenic distillationcolumn having a higher pressure stage and a lower pressure stage,wherein said column is capable of operation in a first mode of operationwherein only liquid nitrogen is produced and a second mode of operationwherein liquid nitrogen and liquid oxygen are produced, to produceliquid nitrogen and liquid oxygen at a first weight ratio comprising thesteps of:using a liquefier to provide a stream of cooled gaseous feedair and a stream of liquefied air; introducing said cooled gaseous feedair into said higher pressure stage of said distillation column forrectification into high pressure nitrogen at the top of said higherpressure stage and a crude oxygen liquid at the bottom of said higherpressure stage; condensing said high pressure nitrogen from said higherpressure stage by heat exchange with an oxygen-enriched liquid from thebottom of said lower pressure stage of said distillation column;utilizing a portion of said condensed nitrogen as reflux to said higherpressure stage; withdrawing the remaining portion of said condensednitrogen as liquid nitrogen product; and operating said column by usingthe first mode of operation for a first period of time and thenoperating said column using the second mode of operation for a secondperiod of time, wherein said first period of time and a said secondperiod of time are sufficient such that when averaged over the combinedfirst and second time periods liquid nitrogen and liquid oxygen areproduced at said first weight ratio wherein:(a) the first mode ofoperation during which only liquid nitrogen is withdrawn as a product isoperated by:(i) introducing at least a portion of at least one of saidliquefied air and said crude oxygen liquid to said lower pressure stageto form lower pressure stage gaseous nitrogen at the top of said lowerpressure stage and said oxygen-enriched liquid at the bottom of saidlower pressure stage; (ii) introducing a stream selected from the groupconsisting of: at least a portion of said crude oxygen liquid, at leasta portion of said oxygen-enriched liquid, at least a portion of saidliquefied air, and mixtures thereof, to a condenser of said lowerpressure stage to condense said lower pressure stage gaseous nitrogen toform a lower pressure stage nitrogen condensate; and (iii) utilizing aportion of said lower pressure stage nitrogen condensate as reflux tosaid lower pressure stage and withdrawing the remaining portion of saidlower pressure stage nitrogen condensate as liquid nitrogen product; and(b) the second mode of operation during which liquid nitrogen and liquidoxygen are withdrawn as products at a second weight ratio of liquidnitrogen to liquid oxygen which is less than or equal to said firstweight ratio is operated by:(i) pressure reducing said crude oxygenliquid and introducing said crude oxygen liquid into said lower pressurestage; (ii) cooling and pressure reducing said stream of liquefied airand introducing said liquefied air into said lower pressure stage at alocation different from the location at which said crude oxygen liquidis introduced into said lower pressure stage; and (iii) operating saidlower pressure stage to produce a low pressure overhead waste streamcontaining nitrogen and said oxygen-enriched liquid which is a productliquid oxygen stream.
 11. The method of claim 10, wherein said secondweight ratio is approximately 1:1.
 12. The method of claim 10, wherein(b) further comprises the step of cooling said liquefied air, theremaining portion of said condensed nitrogen from said higher pressurestage, and said crude oxygen liquid against said low pressure overheadwaste stream containing nitrogen.
 13. The method of claim 10 furthercomprising the steps of:storing an excess amount of liquefied air duringa first time period; and utilizing at least a portion of said excessamount of liquefied air during a second time period.
 14. The method ofclaim 10, wherein:the step of (a)(i) comprises introducing a portion ofsaid liquefied air to said lower pressure stage; and the step of (a)(ii)comprises introducing the remaining portion of said liquefied air, allof said crude oxygen liquid, and all of said oxygen-enriched liquid tosaid condenser of said lower pressure stage.
 15. The method of claim 10,wherein the step of (a)(ii) comprises introducing a portion of saidliquefied air to said condenser of said lower pressure stage.
 16. Themethod of claim 10, wherein the step of (a)(ii) comprises introducing aportion of said crude oxygen liquid to said condenser of said lowerpressure stage.
 17. The method of claim 10, wherein (b) furthercomprises the step of cooling said crude oxygen liquid against saidproduct liquid oxygen.
 18. A cryogenic distillation process forproducing liquid nitrogen including the steps of: (a) liquefying a feedto provide a stream of cooled gaseous feed air and a stream of liquefiedair; (b) rectifying said cooled gaseous feed air in a higher pressurestage of a distillation column into a high pressure nitrogen overheadand a crude oxygen liquid; (c) separating at least a portion of saidliquefied air in a lower pressure stage of said distillation column intolower pressure stage gaseous nitrogen and an oxygen-enriched liquid; (d)condensing said high pressure nitrogen in a reboiler/condenser by heatexchange with said oxygen-enriched liquid to form condensed nitrogen;(e) condensing said lower pressure stage gaseous nitrogen in acondenser; characterized in that said process further comprises thesteps of: (f) introducing a stream selected from the group consistingof: (i) at least a portion of said crude oxygen liquid, (ii) at least aportion of said oxygen-enriched liquid, (iii) at least a portion of saidliquefied air, and (iv) mixtures thereof, to said condenser to condensesaid lower pressure stage gaseous nitrogen to form a lower pressurestage nitrogen condensate and (g) withdrawing said condensed nitrogenfrom said higher pressure stage and said lower pressure stage nitrogencondensate as liquid nitrogen products.
 19. A system for producingliquid nitrogen and liquid oxygen having a liquefier to provide a streamof cooled gaseous feed air and a stream of liquefied air and having adistillation column including: (i) a higher pressure stage forrectifying said cooled gaseous feed air into a high pressure nitrogenoverhead and a crude oxygen liquid; (ii) a lower pressure stage forseparating at least a portion of said cooled liquefied air into lowerpressure stage gaseous nitrogen and an oxygen-enriched liquid; (iii) areboiler/condenser for condensing said high pressure nitrogen by heatexchange with said oxygen-enriched liquid to form condensed nitrogen;and (iv) a condenser for selectively condensing said lower pressurestage gaseous nitrogen, characterized in that:(a) a first set of fluidflow lines and valves extend between the bottom of said higher pressurestage, said condenser, and said lower pressure stage, for permittingcrude oxygen liquid to flow from the bottom of said higher pressurestage to:(i) said condenser during a first mode of operation duringwhich only nitrogen is produced; and (ii) said lower pressure stageduring a second mode of operation during which liquid oxygen and liquidnitrogen are produced; and (b) a second set of fluid flow lines andvalves extend between the bottom of said lower pressure stage, a liquidoxygen product storage, and said condenser, for permitting saidoxygen-enriched liquid to flow from the bottom of said lower pressurestage to:(i) said condenser during said first mode of operation; and(ii) said liquid oxygen product storage during said second mode ofoperation.
 20. The system of claim 19 further comprising a storage tankdisposed between said liquefier and said distillation column for storingan excess amount of said liquefied air.
 21. A system for producingliquid nitrogen and liquid oxygen having a liquefier to provide a streamof cooled gaseous feed air and a stream of liquefied air and having adistillation column including: (i) a higher pressure stage forrectifying said cooled gaseous feed air into a high pressure nitrogenoverhead and a crude oxygen liquid; (ii) a lower pressure stage forseparating at least a portion of said cooled liquefied air into lowerpressure stage gaseous nitrogen and an oxygen-enriched liquid; (iii) areboiler/condenser for condensing said high pressure nitrogen by heatexchange with said oxygen-enriched liquid to form condensed nitrogen;and (iv) a condenser for selectively condensing said lower pressurestage gaseous nitrogen characterized in that:(a) a first set of fluidflow lines and valves extend between the bottom of said higher pressurestage, said condenser, and said lower pressure stage, for permittingcrude oxygen liquid to flow from the bottom of said higher pressurestage to:(i) said condenser during a first mode of operation duringwhich only nitrogen is produced; and (ii) said lower pressure stageduring a second mode of operation during which liquid oxygen and liquidnitrogen are produced; and (b) a second set of fluid flow lines andvalves extend between the bottom of said lower pressure stage, a liquidoxygen product storage, and a vapor waste stream, for permitting:(i) abottom vapor waste stream to flow from a first position near the bottomof said lower pressure stage to said vapor waste stream during saidfirst mode of operation; and (ii) said oxygen-enriched liquid to flowfrom a second position, below said first position, near the bottom ofsaid lower pressure stage to said liquid oxygen product storage duringsaid second mode of operation.